Recently liquid crystal (LC) mixtures have been developed for the realization of flexible substrate based LC displays. These LC mixtures contain reactive polymer precursors that allow the formation of polymer walls in the display, which help to maintain the gap distance of the LC layer. This technology thus enables manufacturing of free form and robust displays by using LC materials.
Free form LC displays can either have a permanent shape other than a rigid flat panel displays, or can even be flexible. The simplest form of the first type are curved TVs that have been developed in the recent past and offer the viewer an enhanced viewing experience. Thereby it is possible to provide displays that are not only shaped in one, but two dimensions, and could be used for example as car dashboards or advertising screens.
Flexible displays, another type of free form displays, have also been developed, and have been proposed for example for use in mobile phones or smart watches utilizing the advantages of flexibility. Further potential applications are foldable or rollable mobile phones, as well as extra-large screens for presentations or home entertainment, which require due to their size to be rollable or foldable for being transported or stowed. Advantageously such devices are based on plastic substrates, instead of rigid glass substrates as used in conventional, unflexible LC displays.
Another display concept, ‘unbreakable’ displays, are also based on plastic substrates and refers to a display design featuring particular robustness, durability, and resistance against mechanical impact. One problem that should be solved is that mobile devices have an elevated risk of being dropped accidentally or becoming otherwise damaged during their normal use. In view of the high value of these devices, a solution to this problem would be highly desirable.
There is thus a great demand for free form or unbreakable LC displays.
One of the main technical challenges of LC displays with flexible substrates is that the LC layer thickness is critical for proper device operation. A proper combination of defined LC layer thickness and LC material properties ensures that the pixels can be switched between a black state and light transmitting state. In case of a varying layer thickness, unwanted interference with the gap distance between the substrates can result in visible optical defects. It should therefore be ensured that the LC layer thickness is not influenced by the bending or the lack of rigidity of flexible plastic substrates.
In conventional LC displays with rigid glass substrates, usually spacer particles are added to the LC layer in order to define and maintain a constant layer thickness. A possible solution for free form displays is to adapt this concept by incorporating supporting structures, like for example polymer walls, that can both resist compression and bind the two substrates together. A suitable manufacturing process is to prefabricate the polymer wall structures, spread the LC mixture on the substrate, and subsequently close the panel with the top substrate. Potential problems with this approach are for example that spreading of the LC mixture is obstructed by the support structures, and that bonding to the top substrate might not be sufficient.
An alternative solution is to create the polymer wall structures by means of a photolithographic process after the display has been assembled. This is schematically illustrated in FIG. 1 showing a polymer wall formation process. FIG. 1(a) shows an LC mixture consisting of LC host molecules (rods), polymerisable monomer (dots), and photo-initiator (not shown). As shown in FIG. 1(b) the LC mixture is filled into the display, or the LC mixture is spread on a first substrate and a second substrate applied on top, and UV radiation (indicated by the arrows) is applied through a photomask. Polymerization induced phase separation takes place, as a result of which polymer walls are formed in irradiated regions according to the mask pattern as shown in FIG. 1(c), while the LC phase of the LC host molecules (rods) in the pixel area is restored.
The principle of creating polymer walls by this method for LC display applications is a known technique that has been described in the literature and has been suggested for use in a variety of display modes.
For example, U.S. Pat. No. 6,130,738 and EP2818534 A1 disclose an LC display that comprises polymer walls formed from one or two polymerisable monomers that are contained in the LC host mixture.
However, the currently used LC mixtures and monomers for use in flexible LC displays with polymer wall formation do still have several drawbacks and leave room for further improvement.
For example, it was observed that the polymerisable compounds and LC media used in prior art do often show insufficient phase separation between the polymer walls and the LC molecules of the LC host mixture. This leads on the one hand to the undesired inclusion of LC molecules in the polymer walls, and on the other hand to increased amounts of polymer molecules dissolved or dispersed in the LC host mixture, both of which can negatively influence the display performance.
Thus, LC molecules trapped in the polymer wall can lead to reduced transparency and contrast of the display, a deterioration of the electrooptical response due to formation of domains with different switching speed, and decreased adhesion of the polymer walls to the substrates. On the other hand, undesired amounts of polymer molecules in the LC host mixture can negatively affect the LC mixture properties.
Moreover, it was observed that the thickness of the polymer walls is often not constant but varying, which can lead to non-uniform pixel size. Besides the polymer walls do often still not show sufficient stability against mechanical pressure on the one hand and sufficient elasticity on the other hand. Also, the polymer walls are often too thick, which reduces transparency and contrast of the display.
It is therefore desirable to have available improved LC mixtures and monomers for use in flexible LC displays which can overcome the drawbacks of materials used in prior art as described above.
The present invention is based on the object of providing novel suitable materials, in particular LC host mixtures comprising polymerisable monomers, for use in flexible LC displays with polymer walls, which do not have the disadvantages indicated above or do so only to a reduced extent.
In particular, the invention is based on the object of providing LC media comprising polymerisable monomers, which enable the formation of polymer walls in a time- and cost-effective manner, and which are suitable for mass production. The formed polymer walls should show clear phase separation from the LC host mixture, without or with a reduced amount of defects or LC molecules trapped in the polymer wall, and without or with a reduced amount of polymer molecules dissolved in the LC host mixture. Also, the polymer walls should show constant thickness, high elasticity, high stability against mechanical pressure, and good adhesion to the substrates.
Another object of the invention is to provide improved LC host mixtures for flexible displays which should show high specific resistance values, high VHR values, high reliability, low threshold voltages, short response times, high birefringence, show good UV absorption especially at longer wavelengths, allow quick and complete polymerisation of the monomers contained therein, and reduce or prevent the occurrence of image sticking in the display.
Another object of the invention is to provide LC dsiplays with polymer walls that show high transparency in the addressed state, good contrast, high switching speed and a large operating temperature range.
Another object of the present invention is to provide an improved technical solution for enabling LCD technology for free form and unbreakable plastic substrate based LC displays.
The above objects have been achieved in accordance with the present invention by materials and processes as described and claimed hereinafter.
Thus, it has surprisingly been found that at least some of the above-mentioned objects can be achieved by using an LC medium which comprises an LC host mixture and one or more polymerisable monomers as disclosed and claimed hereinafter, which contains a polymerisable compound having two different polymerisable groups to the LC medium. It was observed that, by using such an LC medium, it is possible to achieve polymer walls with a more constant thickness and better phase separation between the polymer walls and the LC host mixture.
It has also been surprisingly found that the polymerisable compounds contained in the LC medium can also be used for forming spacers to maintain a constant cell gap between the substrates of the LC display. This can support or even replace the spacer materials that are normally used in prior art.